The present invention relates to a method for generating an ablation program for ablation of material from a surface of a body according to a predetermined desired ablation profile by emitting pulses of a pulsed laser beam onto said surface, to a method for ablating material from a surface of a body according to a predetermined desired ablation profile, to a method for forming control signals to control a laser ablation device for ablating material from a surface of a body according to a predetermined desired ablation profile by means of pulses of a pulsed laser beam emitted by the laser ablation device, and to means for carrying out these methods.
The ablation, i.e. removal, of material from a surface of a body by means of a pulsed laser beam is basically known. During ablation, laser radiation or a laser beam is directed onto the surface to be ablated, where material of the body absorbs at least part of the laser radiation and, if the intensity or the input of energy is sufficient, material is removed from the surface. Therefore, laser ablation can be employed to shape a body in a non-contacting manner, with high precision, in particular even with only small depths of removal.
Various laser ablation methods are known for shaping.
In a variant which is suitable for ablation of a body that is approximately spherical in the region of ablation, laser beam pulses are directed onto the surface, with the target location onto which the respective pulse is to be directed as well as the shape and size of the beam cross-section on the surface being set according to a predetermined ablation program. In many cases, the target location is constant for all pulses and is then not explicitly defined.
In another, particularly important variant, also referred to as “spot scanning” method, material is removed from the surface by guiding a pulsed laser beam over the surface according to a predetermined ablation program. The ablation program is then understood to be a series of target locations on the surface or of corresponding data representing the positions of the target locations onto which at least one pulse of the laser beam is to be respectively directed. If the beam or pulse properties of the used laser radiation are variable, the ablation program can further include at least one indication of a beam or pulse property, particularly defining the energy of the pulse or the fluence, i.e. the energy of the pulse in relation to the irradiated area on a plane which is orthogonal to the direction of the laser beam on the surface of the body. If the laser is working at a constant pulse energy or fluence during ablation, there is no need to provide data representing the pulse energy or fluence for each pulse or target location.
The ablation program is determined on the basis of a predetermined desired ablation profile, i.e. of the definition of desired depths of ablation or depths of removal of the material to be ablated by the pulses as a function of the location on the surface. When generating the ablation program, it is often assumed that each pulse ablates a single-pulse ablation volume which is given by the cross-section of the laser beam at the surface, assumed to be orthogonal to the direction of the beam for this purpose, and by the ablation depth. If several pulses impinge at the same location, the depths of ablation add up so that a greater total depth is achieved. The ablation program is then determined such that by emission of the pulses respective ablation volumes or single-pulse volumes are removed from the surface at the target locations given by the ablation program, so that, on the whole, the desired ablation profile is achieved in the best possible way.
An important field of application of laser ablation according to the so-called “spot scanning” method is the laser ablation of plastic lenses, e.g. contact lenses, or particularly also of corneal tissue in photo-refractive keratectomy or LASIK for correction of defective vision, in particular in the human eye.
In order to obtain as precisely as possible the ablation profile to be achieved, i.e. the desired ablation profile, it is essential that the ablation effect of an individual pulse be well-known when generating the ablation program.
As already mentioned, the process of ablation is determined by the energy per surface area impinging on the surface to be processed or by the effective fluence, in which case, typically, ablation actually occurs only above a material-dependent threshold value for the energy per surface area.
In the case of surfaces which are inclined with respect to the laser beam impinging on them, it has been observed that ablation is less deep than would be expected in the case of orthogonal impingement. This can be explained partially by the fact that the spot produced by the laser beam on the surface has a greater surface area due to the inclination than in the case of perpendicular impingement, whereby the actual fluence on the surface is reduced with respect to the fluence in the case of perpendicular impingement.
WO 01/85075 A1 describes a method for generating a control program according to which a laser spot is passed in a spatially and temporally controlled manner over a cornea that is to be photo-refractively corrected in order to ablate a predetermined desired ablation profile from the cornea. When generating the control program, the influence of the angle between the laser beam and the surface of the cornea on the energy density of the laser spot impinging on the surface of the cornea and/or of that portion of the laser beam energy impinging on the surface of the cornea which is reflected away from the surface is taken into consideration.
WO 01/87201 A1 describes a system for the correction of optical errors in an eye, said system comprising a wavefront analyzer which responds to a wavefront coming from the eye, thereby determining an optical path difference between a reference wave and said wavefront. A converter provides an optical correction on the basis of the path difference and a radially dependent function of ablation efficiency. The correction of ablation efficiency uses a compensating polynomial of the form A+Bρ+Cρ2+Dρ3+ . . . +Xρn, wherein ρ designates a standardized radius which is measured from a central region of the cornea and takes a value of 1 at the outer edge of the region to be corrected. The coefficients of the polynomial are determined by comparing the desired ablation depth with the achieved ablation depth, i.e. by experimentation.
However, the two methods just mentioned leave sufficient space for an improvement in accuracy of the ablation by using an improved ablation program.
When treating defective vision in the human eye by ablation using an excimer laser, the cornea of the eye is shaped such by ablation that a refractive error which causes said defective vision is removed to the largest possible extent. Using conventional methods, the desired refraction can be achieved in approximately 95% of cases with an accuracy of approximately +/−1 diopter.
However, in individual cases there may be problems with night vision and a reduction in contrast sensitivity of twilight vision, which are due to changes in the aspherity of the cornea by the laser surgical treatment. In a healthy eye or an eye which does not have defective vision, the cornea is somewhat prolate and has a negative aspherity with values for the aspherity parameter Q of approximately −0.25. This aspherity compensates for spherical aberrations in the lens of the eye. After laser surgical treatment of near-sightedness, the cornea tends to be somewhat flattened, with the aspherity being substantially greater than in the healthy eye or the eye not having defective vision. These deviations may be at least partially due to the fact that the actual ablation profile achieved by ablation differs from the predetermined desired ablation profile.
For improved ablation, WO 95/27534 describes a method and a system for carrying out photo-refractive keratectomy so as to produce a desired refractive correction in the corneal tissue. Said method and system use control of the effect of liquid on the surface of the cornea in order to reduce the interfering influence of the liquid on the desired ablation process while maintaining the water content of the cornea. It is suggested to control the mean repetition frequency of pulses emitted onto the cornea's surface, so as to reduce an accumulation of liquid between pulses, without dehydrating the cornea, or to select an increased fluence for the pulse emitted onto the cornea's surface so as to reduce the effect of liquid accumulated on the cornea's surface. It is further suggested that, before a pulse intended for ablation is emitted to a site, evaporation energy should be supplied to the latter.
These known methods also still leave room for improving the accuracy of ablation by the use of an improved ablation program.